I have been on a kit buying frenzy lately and amongst others bought some radio kits. These kits are dirt cheap and on Ebay and Aliexpress you can find many different kinds of them. Apparently these kits are used to teach people soldering and manufacturing techniques. Unfortunately those kits teach people how to solder during the 80's and how acceptable manufacturing defects were during those days.
(There are three kits, two are the same AM radio kits, I gave one away)
- The plastic used in these radios is the most crispy kind of plastic you can find on the market. (Note to manufacturers: if your plastic is classified as crispy, that is a bad thing, if you make potato chips, then itis a good thing).
- PCB: the PCB s really quite 80s, pertinax, cardboard, nothing more.
- According to the description on the website the kits are used to show student several engineering and manufacturing techniques. Well, basically they teach students only archaic and backwards production techniques as it is hardly possible even in China to release a product using pertinax board.
Why would I buy these kits?
Despite their shortcomings these kits are great to build together on a rainy Sunday afternoon. Do they required rocket science skill? No, you just need to be precise and also imaginative if you don't speak Chinese, because most of these kits are in Chinese only.
Different kit types
Generally there are two types of kit available on the market. There are kits who are based on a Sony CXA1691 or a clone. Those kits are FM/AM capable. Other kits are based upon a 7 transistor superheterodyne receiver design. Those are AM only. The one I got can be found when you google for HX108, they come in different housings and are generally quite compact.
What do you get?
The kit basically consists of the plastic housing, a bag of bits and a small paper with the schematic and a component listing.
The AM kit
Building the kit is pretty straightforward. The component values are displayed in the schematic alongside the component numbers. Those numbers are then printed on the board. As with all kits start out with the small parts and then mount the bigger ones. There are a few tricky bits you need to be aware off: - The tunable coils all have a different core color. The colors are indicated on the PCB, but only in Chinese. You can use google translate to find out the color names in Chinese, but if you know the resistor color codes you can use the small table at the bottom of the schematic page too.
- There are four disconnects that need to be soldered through. In the schematic they are marked with a small 'x'. They are test points and can be used to troubleshoot the radio in case it does not work.
Once you have inserted and soldered all components it is time power the thing up. Insert two AA batteries and turn the volume button to maximum. You will hear probably nothing. The radio needs to be fine tuned before it will work properly. On the instruction leaflet there are probably instructions telling you how to finetune the damn thing, but I don't understand Chinese. What worked for me was to turn up the volume to maximum then turn each coil until the noise produced was at maximum level. After this procedure I was able to tune in to some radio stations.
(Backside of the instruction leaflet)
(The finished product)
Unfortunately we don't have many AM radio stations around here anymore so this radio is of not much use. Therefore continue to the much more advanced (ahem) AM/FM radio...
AM/FM CXA1691 radio kit
This is the more advanced kit of the two. The radio is designed around the Sony CXA1691, albeit a clone, because the real deal would cost too much of course. The chip is a CD1691CB, a 28-lead SOP device. This is an SMD device, but with a 0.05" pitch which is very doable with a normal soldering iron.
What do you get
The kit consists of a banana yellow housing of nice shiny and crispy plastic, a bag of bits, a leaflet and the speaker, nicely wrapped in newspaper. On the housing it says "CHAO WAI CHA SHI SHOU YIN JI" which is hard to translate, but "shou yin ji" can be translated to radio.
(There is a whole lot of Chinese on the first page)
The schematic in itself is easier to follow than the AM receiver schematic. Basically it is not much more than the recommended layout from the datasheet, no surprises here.
(Look, page two has a schematic!)
There are three tunable coils and two filters. The 455kHz filter is used in the AM circuit and there is a 10.7MHz IF filter for the FM band.
(Component listing and board layout. Note that the documentation was for a different revision. My board did not have the long jumper wire for the power LED. On my board it was routing nicely instead.)
The board itself has component numbers as well as component values. In theory, even without instruction leaflet this kit can be built. Apart from the main IC all components are through hole, just as the AM kit.
(The speaker was nicely packed in a piece of newspaper from one year ago)
(The finished board will look like this. Note, the battery clip is mounted upside down here.)
The board layout is quite messy but function prevails over form here. There are no additional jumper wires needed here and that in itself is quite a feat given the single layer layout.
One thing to keep in mind is placement of the power LED because it has to be placed at the solder side of the board. Also you should figure out how much it should stick out to fit snuggly into the designated hole.
(Finished AM/FM radio. The unit is powered by two D-cell batteries)
Unfortunately I had some issues how to mount the AM antenna. The antenna is your run of the mill graphite antenna with two coils on it and a plastic mounting bracket that is supposed to be screwed over the tuning capacitor. However this gives issues with the tuning wheel getting mis-adjusted and scraping against the plastic of the housing.
What I did was shear off the mounting bracket with a knife and use some hotglue to fixate the antenna to the board. This solved the problem.
(Hotsnot to the rescue!)
The instructions mention (probably) than one of the three tunable coils is for AM, the other two are for FM. Adjusting them does not seem to do much. But this is something I am going to investigate further...
Both radios are a bit finicky to tune correctly. This is due to the small tuning wheel which makes fine tuning a but of a nuisance. The AM radio has a very low output power. It has enough if you are sitting down in a quiet room. Besides the power there are not many radio stations active anymore on AM these days where I live so its use is limited.
The FM unit will probably end up in my shed because it is quite capable of receiving FM radio stations and it produces enough volume to be useful when I am working there. And because it is powered from two D-cell batteries I can probably use it without it running out of juice for years.
Looking back I actually enjoyed building these kits. The internals very much look like they were designed in the 1980's and 1990's. The cheap board, single layer design all contribute to this.